This invention relates to orthodontic appliances, particularly to orthodontic appliances that include a polycrystalline alumina-based ceramic material that has a relatively small grain size yet generally high translucency.
Although performance and durability are highly desirable characteristics of orthodontic brackets, for example, they alone are not the sole concern for practitioners and patients. Aesthetic value, or how orthodontic materials look inside the mouth is just as desirable.
For orthodontic devices (typically, brackets, which are small slotted bodies for holding a curved arch wire, and associated tooth bands if banded attachment is used), stainless steel is an ideal material because it is strong, nonabsorbent, weldable, and relatively easy to form and machine. A significant drawback of metal appliances, however, relates to cosmetic appearance when the patient smiles. Adults and older children undergoing orthodontic treatment are often embarrassed by the xe2x80x9cmetallic smilexe2x80x9d appearance of metal bands and brackets, and this problem has led to various improvements in recent years.
One area of improvement involves use of nonmetal materials. Both plastic and ceramic materials present an improved appearance in the mouth, and often the only significantly visible metal components are thin arch wires that are cosmetically acceptable. Plastic is not an ideal material because it lacks the structural strength of metal, and is susceptible to staining and other problems. Ceramics such as sapphire or other transparent crystalline materials have undesirable prismatic effects. Also, single crystal aluminum oxide appliances are subject to cleavage under the loads that occur in the course of orthodontic treatment. Other ceramics have been largely opaque so that they either do not match tooth color or require coloring.
U.S. Pat. No. 4,954,080 (Kelly et al.) describes orthodontic appliances made from a polycrystalline ceramic material such as alumina. The ceramic material of the appliances described in this patent transmits sufficient light to enable the appliance to pick up the color of the tooth. While the appliances described are considered satisfactory, there is a need in the art for orthodontic appliances made of a ceramic material that is stronger. In such an appliance, it would also be advantageous to have a material with no pre-determined color and the ability to blend with or color-match the adjacent dentition.
The present invention is directed to orthodontic appliances that include a polycrystalline ceramic material that has a desirable translucent quality. This is particularly advantageous for use in orthodontic appliances to achieve further cosmetic improvement by having a translucent quality that picks up the color of the underlying tooth to make the orthodontic appliance blend with the tooth. Preferably, the material has a Contrast Ratio value of less than about 0.7.
The polycrystalline translucent ceramic material is formed by pressing a powder material, such as high purity aluminum oxide, into a desired shape, sintering the shaped material to provide closed porosity, and subjecting the sintered material to hot isostatic pressing to yield a single phase material having substantially zero porosity and an average grain size of no greater than 1.0 micrometer (i.e., micron). This small grain size contributes to a stronger material than conventional ceramic materials without detrimentally effecting translucency. This is surprising as small grain size is often considered to prevent relatively high translucency. Preferably, the material has a flexure strength of at least about 400 MPa.
Thus, in one embodiment the present invention provides an orthodontic appliance that includes a polycrystalline translucent aluminum oxide ceramic material having an average grain size of no greater than 1.0 micron and a Contrast Ratio value of less than about 0.7.
The present invention also provides a kit that includes a plurality of orthodontic appliances, wherein at least one of the appliances includes a polycrystalline translucent aluminum oxide ceramic material having an average grain size of no greater than 1.0 micron and a Contrast Ratio value of less than about 0.7. The kit preferably further includes a component selected from the group consisting of an orthodontic adhesive, an adhesive primer, an appliance positioning tool, and combinations thereof.
The present invention also provides a method for making an orthodontic appliance or appliance perform that includes a polycrystalline translucent aluminum oxide ceramic material having a grain size of no greater than 1.0 micron and a Contrast Ratio value of less than about 0.7. The method includes: providing an aluminum oxide powder; forming the powder into an article having a desired shape; sintering the shaped article to obtain a sintered article having closed porosity; and subjecting the sintered article to hot isostatic pressing to further densify and form an orthodontic appliance or appliance perform comprising polycrystalline translucent aluminum oxide ceramic material having a grain size of no greater than 1.0 micron and a Contrast Ratio value of less than about 0.7. The method preferably further includes deagglomerating the aluminum oxide powder prior to forming the powder into an article having a desired shape. This is preferably accomplished by subjecting the aluminum oxide powder to ultra-sonication.